Why isn't Linux on more desktops? Here are the usual reasons: 1) Microsoft has hardware vendors locked-in; 3) Monstrous Windows installed base; and 3) Operating system and application FUD. Here's the reason we don't talk about much: the Linux distributors don't encourage the Linux desktop.
Oh, there are lots of Linux desktops. You can read my reviews, such as my comparison of Fedora 10; openSUSE 11.1; and Ubuntu 8.10 or my look at Debian 5's five best features. And, I just touch the surface of Linux distributions. There are hundreds of Linux distributions listed in DistroWatch, and most of them are desktops.
So, what's the problem? How many of those desktops are actually supported and advertised by their vendors or groups? I count two. They are Novell with its SLED (SUSE Linux Enterprise Desktop) 11 and Canonical's Ubuntu on Dell computers like the Dell Mini 9.